1. Field of the Invention
The present invention relates to a device for reducing electrostatic discharge (ESD) damage in thin film read heads which enables measurement of gap resistances and, more particularly, to such a device and method wherein the resistance of first and second gap layers can be measured in parallel or the resistance of each of the first and second gap layers can be measured separately.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and and actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance GMR read head employs a spin valve sensor for sensing the magnetic field signals from the rotating magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The sensor and the first and second leads are located between first and second dielectric read gap layers which are, in turn, located between ferromagnetic first and second shield layers. Accordingly, the GMR head is electrically isolated from the two shields by the first and second gap layers which are typically aluminum oxide (Al2O3). The gap length, which is the distance between the shield layers, is continually being shortened in order to achieve higher areal density. For a given sensor thickness, therefore, the gap layers have to become thinner. In head designs, the shields are typically not electrically connected to any other conductors on the slider, and are electrically isolated from each other. As a result, a charge may accumulate on the shields during processing. The presence of this charge causes a potential difference across the gap layers. When this voltage reaches a sufficiently high value, the dielectric breaks down, and electrical shorts can occur at the location of the breakdown. This is a type of electrostatic discharge (ESD) damage. Shorts between the sensor and the shields are detrimental to the operation of the head. A typical specification on the resistance between the shields and the sensor is 100 kOhms. Accordingly, any head with a resistance less than 100 kOhms between the read sensor and either shield fails such a test. Losses at wafer final test due to shield shorts can be as high as 30%. One way to prevent the charging of the shields is to electrically short both shields to one side of the sensor via a lead and then remove the short during slider fabrication. While this will provide protection against process-induced charging, it does not allow the ability to test for shield shorts due to other phenomena, such as pinholes in the gap dielectric.
The present invention provides a device and method of reducing ESD damage to the sensor of the read head while enabling measurement of the first and second gap resistances. The first read gap layer can be considered to have a resistance RG1 between the first shield layer and one of the first and second lead layers and the second read gap layer can be considered to have a resistance RG2 between the second shield layer and one of the first and second lead layers. A short is provided via a plurality of resistors between a first node and each of the first and second shield layers wherein the plurality of resistors includes at least first and second resistors RS1 and RS2 and the first node is connected to either one of the first and second leads. A second node is located between the first and second resistors RS1 and RS2. An operational amplifier has first and second inputs connected to the first and second nodes respectively so as to be across the first resistor RS1 and has an output connected to the first node for maintaining the first and second nodes at a common voltage potential.
In one embodiment of the invention the first and second shield layers are shorted together. In this embodiment a test instrument can be employed for determining the combined parallel resistance of the first and second gap layers by having a first side of the test instrument connected to the first node and a second side connected to each of the first and second shield layers. In another embodiment of the invention the second resistor RS2 is connected between the second node and the shield layer and a third resistor RS3 is connected between the second node and the first shield layer. In this embodiment the test instrument has a first side connected to the first node and a second side connected to the first shield layer for determining the resistance of the first gap layer separately. Alternatively, the test instrument can be employed with its first side connected to the first node and its second side connected to the second shield layer so that the resistance of the second gap layer can be determined separately. In another aspect of the invention the sensor and the resistors RS1 and RS2 or RS1, RS2 and RS3 are coplanar. This is accomplished by forming a layer of sensor material on a wafer and then patterning the layer of material to individually form the sensor and each of the resistors. The formation of the sensor material layer can be by sputter deposition and the patterning may be accomplished by photolithography.
An object of the present invention is to reduce ESD damage to the sensor of a read head while enabling measurement of gap resistances in parallel or separately.
Another object is to accomplish the foregoing object with the sensor and a plurality of resistors patterned from a common material layer wherein the plurality of resistors are in parallel with the resistances of the first and second gap layers.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.